library(ncdf4)

# files are 229x161x8760
FILE_1 = nc_open('/emhires-data/data-warehouse/gridded/ERA5/era5-hourly-100m_v_component_of_wind-2018.europe.nc')
FILE_2 = nc_open('/emhires-data/data-warehouse/gridded/ERA5/era5-hourly-10m_v_component_of_wind-2018.europe.nc')

lat = ncvar_get(FILE_1, 'latitude')
lon = ncvar_get(FILE_1, 'longitude')

data_1 = ncvar_get(FILE_1, 'v100') # 2.5 GB in memory
data_2 = ncvar_get(FILE_2, 'v10')

nc_close(FILE_1)
nc_close(FILE_2)

cor_value = matrix(NA, ncol = length(lat), nrow = length(lon))
pb = txtProgressBar(max = length(lon), style = 3)
for (i in seq(1, length(lon))) {
  setTxtProgressBar(pb, i)
  for (j in seq(1, length(lat))) {
    cor_value[i,j] = cor(data_1[i,j,], data_2[i,j,])
  }
}
close(pb)